Various methods for preparing oxy-diphthalic anhydrides have been described in literatures. In the old time, there was a method based on an oxidation reaction of tetramethylphenyl ether proposed by Marvel et al. (J. Am. Chem. Soc. 80, 1197-(1955)). This oxidation reaction is inappropriate from the view point of the industrial preparation, since potassium permanganate with high toxicity is employed as an oxidant. Further, in Japanese Provisional Patent Publication No. 122738/1978, there has been described a method for obtaining oxy-diphthalic anhydrides by reacting a halogenated (halo-) phthalic anhydride with caustic soda in a polar solvent, but the yield of the product prepared by this reaction is extremely low. Japanese Provisional Patent Publication Nos. 136246/1980 and 127343/1980 describe a method for preparing oxy-diphthalic anhydrides by reacting a nitro-phthalic anhydride with sodium nitrite and a carbonate salt in a polar solvent. These methods are hardly used for industrial purposes since a harmful nitric oxide is by-produced during the reaction.
In Japanese Provisional Patent Publication No. 96183/1988, there has been disclosed a method for preparing oxy-diphthalic anhydrides by reacting a halo-phthalic anhydride with potassium carbonate and potassium fluoride or cesium fluoride in a polar solvent. However, this method is based on the reaction in which water participates, and can not provide a sufficient yield because of the low reaction ratio caused by the existence of water.
Japanese Patent Publication No. 50071/1991 discloses a method for obtaining oxy-diphthalic anhydrides by dehydrohalogenating a hydroxy-phthalic anhydride and a halo-phthalic anhydride in the presence of potassium carbonate. However, this method is required to prepare the hydroxy-phthalic anhydride by hydrolysis of a halo-phthalic anhydride, thus it can not be said to be an industrially advantageous preparation method.
In Japanese Provisional Patent Publication No. 254673/1989, there has been disclosed a method for directly preparing oxy-diphthalic anhydrides directly by reacting a halo-phthalic anhydride with potassium carbonate in the presence of a potassium fluoride catalyst. This method requires to stir carefully because this method is based on the neat reaction or the similar reaction thereto, thus it is difficult to allow the reactivity to stabilize. In addition, in this method, potassium carbonate can be used restrictedly, and when another carbonate salt such as sodium carbonate is employed in place of potassium carbonate in this method, the reactant shows a high viscosity and the reaction cannot be completed because the reactant does not show any sufficient flowability.